Autotaxin (ATX, ENPP2) is a secreted glycoprotein widely present in biological fluids, including blood, cancer ascites, synovial, pleural and cerebrospinal fluids, originally isolated from the supernatant of melanoma cells as an autocrine motility stimulation factor (Stracke, M. L., et al. Identification, purification, and partial sequence analysis of autotaxin, a novel motility-stimulating protein. J Biol Chem 267, 2524-2529 (1992), which is incorporated by reference in its entirety). ATX is encoded by a single gene on human chromosome 8 (mouse chromosome 15) whose transcription, regulated by diverse transcription factors (Hoxal3, NFAT-1 and v-jun), results in four alternatively spliced isoforms (α, β, γ and δ). See, for example, Giganti, A., et al Murine and Human Autotaxin alpha, beta, and gamma Isoforms: Gene organization, tissue distribution and biochemical characterization. J Biol Chem 283, 7776-7789 (2008); and van Meeteren, L. A. & Moolenaar, W. H. Regulation and biological activities of the autotaxin-LPA axis. Prog Lipid Res 46, 145-160 (2007); Hashimoto, et al, “Identification and Biochemical Characterization of a Novel Autotaxin Isoform, ATXδ,” J. of Biochemistry Advance Access (Oct. 11, 2011); each of which is incorporated by reference in its entirety.
ATX is synthesized as a prepro-enzyme, secreted into the extracellular space after the proteolytic removal of its N-terminal signal peptide (Jansen, S., el al Proteolytic maturation and activation of autotaxin (NPP2), a secreted metastasis-enhancing lysophospho lipase D. J Cell Sci 118, 3081-3089 (2005), which is incorporated by reference in its entirety). ATX is a member of the ectonucleotide pyrophosphatase/phosphodiesterase family of ectoenzymes (E-NPP) that hydrolyze phosphodiesterase (PDE) bonds of various nucleotides and derivatives (Stefan, C, Jansen, S. & Bollen, M. NPP-type ectophosphodiesterases: unity in diversity. Trends Biochem Sci 30, 542-550 (2005), which is incorporated by reference in its entirety). The enzymatic activity of ATX was enigmatic, until it was shown to be identical to lysophospholipase D (lysoPLD) (Umezu-Goto, M., et al. Autotaxin has lysophospholipase D activity leading to tumor cell growth and motility by lysophosphatidic acid production. J Cell Biol 158, 227-233 (2002), which is incorporated by reference in its entirety), which is widely present in biological fluids. Since ATX is a constitutively active enzyme, the biological outcome of ATX action will largely depend on its expression levels and the local availability of its substrates. The major lysophospholipid substrate for ATX, lysophosphatidylcholine (LPC), is secreted by the liver and is abundantly present in plasma (at about 100 μM) as a predominantly albumin bound form (Croset, M., Brossard, N., Polette, A. & Lagarde, M. Characterization of plasma unsaturated lysophosphatidylcholines in human and rat Biochem J 345 Pt 1, 61-67 (2000), which is incorporated by reference in its entirety). LPC is also detected in tumor-cell conditioned media (Umezu-Goto, M., et al.), presumably as a constituent of shed microvesicles. ATX, through its lysoPLD activity converts LPC to lysophosphatidic acid (LPA).
LPC is an important inflammatory mediator with recognized effects in multiple cell types and pathophysiological processes. It is a major component of oxidized low density lipoprotein (oxLDL) and it can exist in several other forms including free, micellar, bound to hydrophobic proteins such as albumin and incorporated in plasma membranes. It is produced by the hydrolysis of phosphatidylcholine (PC) by PLA2 with concurrent release of arachidonic acid and in turn of other pro-inflammatory mediators (prostaglandins and leukotrienes). Moreover, LPC externalization constitutes a chemotactic signal to phagocytic cells, while interaction with its receptors can also stimulate lymphocytic responses. LPC has been shown to have therapeutic effects in experimental sepsis, possibly by suppressing endotoxin-induced HMGB1 release from macrophages/monocytes.
LPA, the product of ATX action on LPC, is a bioactive phospholipid with diverse functions in almost every mammalian cell line (Moolenaar, W. H., van Meeteren, L. A. & Giepmans, B. N. The ins and outs of lysophosphatidic acid signaling. Bioessays 28, 870-881 (2004), which is incorporated by reference in its entirety). LPA is a major constituent of serum bound tightly to albumin, gelsolin and possibly other as yet unidentified proteins. (See, e.g., Goetzl, E. J., et al Gelsolin binding and. cellular presentation of lysophosphatidic acid. J Biol Chem 275, 14573-14578 (2000); and Tigyi, G. & Miledi, R, Lysophosphatidates bound to serum albumin activate membrane currents in Xenopus oocytes and neurite retraction in PC 12 pheochromocytoma cells. J Biol Chem 267, 21360-21367 (1992); each of which is incorporated by reference in its entirety.)
LPA is also found in other biofluids, such as saliva and follicular fluid, and has been implicated in a wide array of functions, such as wound healing, tumor invasion and metastasis, neurogenesis, myelination, astrocytes outgrowth and neurite retraction. The long list of LPA functions was also explained with the discovery that it signals through G-protein coupled receptors (GPCRs), via classical second messenger pathways. Five mammalian cell-surface LPA receptors have been identified so far. The best known are LPA1-3 (namely Edg-2, Edg-4 and Edg7) which are all members of the so-called ‘endothelial differentiation gene’ (EDG) family of GPCRs (Contos, J. J., Ishii, I. & Chun, J. Lysophosphatidic acid receptors. Mol Pharmacol 58, 1188-1196 (2000), which is incorporated by reference in its entirety). LPA receptors can couple to at least three distinct G proteins (Gq, Gi and G12/13), which, in turn, feed into multiple effector systems. LPA activates Gq and thereby stimulates phospholipase C (PLC), with subsequent phosphatidylinositol-bisphosphate hydrolysis and generation of multiple second messengers leading to protein kinase C activation and changes in cytosolic calcium. LPA also activates Gi, which leads to at least three distinct signaling routes: inhibition of adenylyl cyclase with inhibition of cyclic AMP accumulation; stimulation of the mitogenic RAS-MAPK (mitogen-activated protein kinase) cascade; and activation of phosphatidylinositol 3-kinase (PI3K), leading to activation of the guanosine diphosphate/guanosine triphosphate (GDP/GTP) exchange factor TIAM1 and the downstream RAC GTPase, as well as to activation of the AKT/PKB antiapoptotic pathway. Finally, LPA activates G12/13, leading to activation of the small GTPase RhoA, which drives cytoskeletal contraction and cell rounding. So, LPA not only signals via classic second messengers such as calcium, diacylglycerol and cAMP, but it also activates RAS- and RHO-family GTPases, the master switches that control cell proliferation, migration and morphogenesis.
LPA signaling through the RhoA-Rho kinase pathway mediates neurite retraction and inhibition of axon growth. Interfering with LPA signaling has been shown to promote axonal regeneration and functional recovery after CNS injury or cerebral ischemia. (See Broggini, et al., Molecular Biology of the Cell (2010), 21:521-537.) It has been reported that addition of LPA to dorsal root fibers in ex vivo culture causes demyelination, whereas LPC fails to cause significant demyelination of nerve fibers in ex vivo cultures without further addition of recombinant ATX to the culture which when added caused significant demyelination at equivalent levels to LPA presumable due to conversion of LPC to LPA through the enzymatic activity of ATX. Moreover, injury induced demyelination was attenuated by about 50% in atx+/− mice (Nagai, et al., Molecular Pain (2010), 6:78).
A number of diseases or disorders involve demyelination of the central or peripheral nervous system which can occur for a number of reasons such as immune dysfunction as in multiple sclerosis, encephalomyelitis, Guillain-Barre Syndrome, chronic inflammatory demyelinating polyneuropathy (CIDP), transverse myelitis, and optic neuritis; demyelination due to injury such as spinal cord injury, traumatic brain injury, stroke, acute ischemic optic neuropathy, or other ischemia, cerebral palsy, neuropathy (e.g. neuropathy due to diabetes, chronic renal failure, hypothyroidism, liver failure, or compression of the nerve (e.g. in Bell's palsy)), post radiation injury, and central pontine myelolysis (CPM); inherited conditions such as Charcot-Marie-Tooth disease (CMT), Sjogren-Larsson syndrome, Refsum disease, Krabbe disease, Canavan disease, Alexander disease, Friedreich's ataxia, Pelizaeus-Merzbacher disease, Bassen-Kornzweig syndrome, metachromatic leukodystrophy (MLD), adrenoleukodystrophy, and nerve damage due to pernicious anemia; viral infection such as progressive multifocal leukoencephalopathy (PML), Lyme disease, or tabes dorsalis due to untreated syphilis; toxic exposure due to chronic alcoholism (which is a possible cause of Marchiafava-Bignami disease), chemotherapy, or exposure to chemicals such as organophosphates; or dietary deficiencies such as vitamin B12 deficiency, vitamin E deficiency and copper deficiency. Other demyelination disorders may have unknown causes or multiple causes such as trigeminal neuralgia, Marchiafava-Bignami disease and Bell's palsy. One particularly successful approach to treating demyelination disorders which are caused by autoimmune dysfunction has been to attempt to limit the extent of demyelination by treating the patient with immunoregulatory drugs. However, typically this approach has merely postponed but not avoided the onset of disability in these patients. Patients with demyelination due to other causes have even fewer treatment options. Therefore, the need exists to develop new treatments for patients with demyelination diseases or disorders.